Contrary to what seismologists do, our work here at the NASA Ames Research Center concentrates on the time before major earthquakes.
We try to fully understand the fundamental physical processes that take place in rocks deep in the Earth prior to seismic events.
These processes are driven by the ever increasing stress, which the rocks experience under the onslaught of gigantic tectonic forces (Stress = force per unit cross section). Tectonic forces cause continental and oceanic plates to move, to slide past each other or to collide. Rocks deep in the Earth contain dormant electronic charge carriers, e.g. pairs of ions that are tightly bonded together and therefore electrically inactive.
These dormant electronic charge carriers are so inconspicuous that, until recently, nobody knew that they even existed.
They hold the key to understand what happens before earthquakes. High levels of stress cause rocks to plastically deform. Plastic deformation proceeds by way of atoms/ions in the crystal structure of the mineral grains moving one by one in a zipper-like fashion.
This process is described as "dislocation" movement. The moving dislocations cause the dormant electronic charge carriers to "wake up". In other words, when dislocations sweep through mineral grains under stress, the rock "charges up" like a battery and becomes a source of electric current.
What is special is the recognition that an electric current can flow out of the stressed rock and spread into the surrounding unstressed rock. This current is carried by a particular form of electronic charge carriers known as defect electrons or holes.
Because they are so special they are called positive holes or "pholes" for short. The pholes can propagate through kilometers of rocks. They pass through sand and soil. What interests us in the context of ionization of the air at the rock surface is the electric field which the pholes build up at the rock surface.
In a theoretical 1984 paper (in Physical Review B) we calculated this field and predicted that it would reach 1,000,000 Volt per centimeter. Such high electric fields are enough to ionize the air that is in contact with the rock surface.
Now, 24 years later, we have been able to demonstrate experimentally that this ionization indeed takes place.
We can show that, when we squeeze the rock to near its breaking point, airborne ions are formed at the rock surface at a tune of 1,000,000,000 (one billion) ions per square centimeter per second and more. This is a process never before observed.
In fact, seismologists (who seldom have enough physics training to follow this train of arguments or read theoretical papers) will have the tendency to say: "This can't be true."
Some will say that the ionization is due to radioactive radon coming out of the ground. Radon of course also causes ionization but its release is spotty and episodic at best. Also the level of ionization in the air caused by radon is generally orders of magnitude less than what can be produced by "our" process. If this stress-induced ionization process occurs in nature (I have every reason to believe that it will occur under the right circumstances), billions of ions in the air will be available to act as condensation nuclei for tiny water droplets (provided the relative humidity in the air is in the suitable range).
Where this droplet formation occurs, close to the surface of the Earth or higher up in the atmosphere, will determine whether we are talking about fog or about clouds. Of course, if the relative humidity in the air is too low, droplet condensation may not occur and no fog or clouds will form. Therefore, not every earthquake will be preceded by fog/cloud formation. However, if fog/cloud formation occurs together with other indicators that I have not mentioned here, it can be counted as a pre-earthquake signal.
Prof.
adj. Dr
Freund Friedemann
NASA Ames Research Center, MS 242-4
MOFFETT FIELD, CA 94035-1000
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Department of Physics
San Jose State University
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Principal Investigator
Carl Sagan Center (CSC), SETI Institute (USA)